1. Field of the Invention
The present invention relates to a deterioration signal generation device for a gas sensor which simulates a detection signal (deterioration signal) that is output by a gas sensor in a deteriorated state of the same, a gas sensor detecting the fuel-air ratio of exhaust gas from an internal combustion engine based on the concentration of particular components in the exhaust gas.
2. Description of the Related Art
Gas sensors are well-known. They are mounted in an exhaust gas passage of an internal combustion engine such as an automobile engine to detect the fuel-air ratio of the exhaust gas based on the concentration of particular components in the exhaust gas. A sensor element of such a gas sensor detects the fuel-air ratio of exhaust gas utilizing the fact that the magnitude of a current flowing the element itself or a voltage at the element undergoes binary or linear changes in accordance with the concentration of the particular element in the exhaust gas. A detection signal output from the gas sensor is transmitted to an ECU (electronic control unit), and the ECU exercises feedback control of the fuel-air ratio such as adjustment of the fuel injection amount in the engine based on the detection signal thus received.
In such a gas sensor, the sensor element is deteriorated due to aging after use for a long time because it is exposed to exhaust gas in an exhaust gas passage. Referring to the development of an ECU under the circumstances a design that allows parameters for feedback control over a fuel-air ratio to be determined optimally even for a detection signal obtained from a gas sensor in a deteriorated state is adopted such that the accuracy of feedback control over the fuel-air ratio can be maintained even when the gas sensor is deteriorated to some degree. For example, accelerated life testing is carried out to provide gas sensors at different levels of deterioration. Transient states of deterioration signals obtained from those gas sensors are predicted from the detection signals and signal detected from a normal gas sensor, whereby parameters for control are set.
When an actual vehicle is tested to check the state of cleaning of exhaust gas, a gas sensor which has been deteriorated through accelerated life testing as described above is mounted in the actual vehicle in order to check whether feedback control over the fuel-air ratio is properly exercised by an ECU even with a deteriorated gas sensor. However, it is difficult to fabricate each of gas sensors as intended through accelerated life testing such that the sensors reproduce a plurality of states of deterioration to be used for such tests. Further, it is troublesome to switch the gas sensors in different states of deterioration each time a test is conducted. Under the circumstance, deterioration signal generation devices (deterioration simulators) capable of simulating deterioration signals from gas sensors have been developed (for example, see JP-A-2004-93957).
Such a deterioration signal generation device is interposed between a normal gas sensor (in other words, a gas sensor to serve as a reference for the generation of deterioration signals) mounted in an actual vehicle and an ECU. The device processes a detection signal input to the same to simulate a deterioration signal and outputs the deterioration signal to the ECU. Specifically, a deterioration signal generation device disclosed in JP-A-2004-93957 generates a deterioration signal on a simulated basis by changing the gain of a detection signal or delaying response characteristics of the detection signal.
3. Problems to be Solved by the Invention
In most cases, a gas sensor is mounted near a cleaner that is provided downstream of an exhaust gas passage. Therefore, when a target fuel-air ratio for a mixture supplied to the internal combustion engine is changed through adjustment of the fuel injection amount, it takes some time for the mixture at the new fuel-air ratio to reach the gas sensor after being burned in the engine (combustion chamber). For this reason, there is a time lag (delay) between the timing when a change is made to the target fuel-air ratio of the mixture and the timing when a change starts appearing in a detection signal output from the gas sensor.
When a gas sensor is used for a long time, a protector of the gas sensor (which is specifically a protector with a gas communication hole covering the periphery of the sensor element) may be subjected to deterioration with time such as clogging of the gas communication hole. When the gas sensor is deteriorated as thus described, the replacement of the gas in the protector through the gas communication hole becomes slow. Thus, a detection signal from the gas sensor takes a longer time (delay time) to start changing as a result of a change in the target fuel-air ratio of the mixture when compared to a detection signal from a gas sensor which is not deteriorated. For this reason, when an ECU is developed taking deteriorated states of a gas sensor into consideration, an ECU capable of promoting more accurate feedback control over fuel-air ratio cannot be developed without paying attention to the fact that a delay time as described above changes (increases) as time passes. However, it has not been possible to simulate a deterioration signal from a gas sensor with such a delay time varied with the deterioration signal generation device disclosed in JP-A-2004-93957, although the device can simulate a deterioration signal with variable gain and response characteristics.
The invention has been made to solve the above-described problem, and it is an object of the invention to provide a deterioration signal generation device for a gas sensor which detects the fuel-air ratio of exhaust gas from an internal combustion engine based on the concentration of particular components in the exhaust gas, the device being capable of simulating a deterioration signal output by the gas sensor and outputting the deterioration signal with some delayed.